Following up on my previous post, a few more points are worth making
regarding the scientific process.
First, regarding uncertainty. Earth's atmosphere and oceans do form a
more complicated system than the simple model I described. For
example, here's one way in which it is possible that temperatures
would not rise much in response to carbon dioxide impeding the outward
flow of heat. When temperatures go up initially, that means more
water vapor in the atmosphere. If that water vapor condenses into
clouds, the extra clouds could reflect enough sunlight back into space
to reduce the heating and make temperatures fall back to normal. This
mechanism would act like a thermostat keeping Earth's surface within a
narrow temperature range, and we wouldn't need to worry about keeping
our carbon emissions in check. So, if you heard Arrhenius's warming
prediction in 1896, you could easily say, "but there's a lot of
uncertainty in that prediction because we don't understand cloud
formation. Maybe there won't be that much warming. It's uncertain."
The point I want to make is that uncertainty cuts both ways. Water
vapor is itself a greenhouse gas, so if the extra vapor does not
condense into clouds, the greenhouse warming will be accelerated.
Yes, the prediction is uncertain....but that means that more extreme
outcomes, as well as less extreme outcomes, are possible.
If a little bit of warming produces clouds which shut down further
warming, we would call that a negative feedback loop; negative because
any change contains the seeds of its own reversal. If instead a bit
of warming creates water vapor which accelerates the warming, we would
call that a positive feedback loop; positive because a little movement
encourages further movement in the same direction. One reason climate
is complicated is that it is full of feedback loops, another example
being that reduced ice coverage causes more sunlight to be absorbed,
which reduces ice coverage further, etc. So what's the verdict on the
cloud formation? We still don't know; it may depend on how much
small-particle pollution we produce, because these small particles
provide the seeds for cloud condensation. But meanwhile, temperatures
keep rising. So while we puzzle over the details, let's not forget
the big picture: we keep making Earth's carbon-dioxide blanket thicker
and thicker.
Second point: I've repeatedly stressed the important of models in terms of
understanding a system. Models are great for exploring a variety of
scenarios, but is there anything we can say about climate that does
not depend on what model we adopt? Such model-independent statements
can be valuable anchors when we're not sure which model to adopt. I'd
like to focus particularly on a (more or less) model-independent
statement regarding sea levels. We can get rid of models and just
accumulate data regarding sea levels and carbon dioxide levels in the
past, and then we can simply ask, what is the typical sea level when
the carbon dioxide level is 400 parts per million, as we have now
caused it to be? (It's up from about 275 before the Industrial
Revolution.)
The answer is shocking: 24 meters, or 80 feet! Go ahead and play with
this interactive flood map to see what such a rise will do to your
state or country.
Now I have to give a few caveats. First, changes in carbon dioxide
concentration and sea levels occurred very slowly in the past.
Although we are pumping carbon dioxide in very quickly, it's quite
likely that it will be hundreds or even a few thousand years before
the effects of the carbon input are fully realized and sea levels rise
this much. Essentially no one is predicting these sea levels within
our children's lifetimes. But still....this will be a lot for our
great-great-grandchildren to deal with. And yes, there's uncertainty on this
prediction. Sea levels may rise less than this. But they may also rise more
than this.
Second caveat: this prediction is not entirely model-independent. To
be an extreme devil's advocate, if CO2 levels in the past were somehow
a natural effect of higher sea levels rather than a cause, then we could not
use past data to predict what would happen when we artificially increase
CO2 levels today. To be clear, I invoke that scenario not because I
believe it, but simply to highlight how an apparently
model-independent statement is often not entirely
model-independent. If all kinds of crazy models are allowed into the
discussion, then very few truly model-independent statements can be
made. But within the scope of "reasonable" models, we can say that
sea levels will rise by around 24 meters; we just don't know long
that will take. Predicting how long it will take requires a model!
If you are interested in further reading, start with this Skeptical
Science post, which summarizes this publication in an approachable
way. Skeptical Science, by the way, is a good resource for rebutting
common climate myths.
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